The stiffness and damping characteristics of a flexible structure are usually fixed parameters which cannot be easily changed or controlled once fabrication of the structure is complete. Flexible structures which are used in dynamic mechanical systems may experience a wide range of input forces and it therefore may be necessary to adjust the response of the structure to satisfy certain operating requirements. For example, engineering applications in which it may be desirable to adaptively control the mechanical properties of a structure include, inter alia, the control of sound propagation and vibrations in aerospace and automotive applications, and flexible fixturing in advanced manufacturing and robotic manipulator systems. The variable control of structural behavior which is contemplated herein is to be distinguished from traditional damping arrangements, which typically operate as discrete couplings between portions of a structure, or between the structure and the source of mechanical disturbance. While a large variety of known damping systems can successfully control the motion of coupled members in many cases, discrete dampers are inherently inadequate to control the overall constitutive characteristics of flexible structures subjected to input disturbances or forces.
One approach to the control of the overall dynamic properties of a flexible structure which represents an improvement over localized damping systems is described in Hubbard, Jr., U.S. Pat. No. 4,565,940. A viscoelastic layer of material is applied to the flexible structure and is constrained by a piezoelectric film. The effect of the layer is to dissipate input vibrations or forces for improved damping characteristics. The damping effect can be variably controlled by adjusting the application of a voltage potential to the piezoelectric film to change its stiffness. However, active control of the mechanical properties of a structure using piezoelectric films and viscoelastic materials include numerous shortcomings. The viscoelastic material itself has static properties. Piezoelectric materials are expensive and generally are incapable of sustaining sufficient forces to allow for the adequate control of structures in most practical systems.
It may therefore be desirable to employ an electroactive fluid as a component of a flexible, composite structure to provide for variable and controllable mechanical properties. Electroactive fluids consist of suspensions of very fine particles in a dielectric liquid media. Electroactive fluids experience changes in their physical properties in the presence of an electric field, and for this reason are useful in a wide variety of mechanical treatments. One type of electroactive fluid is what is known as an electrorheological or "electroviscous" fluid. Electrorheological fluids are electroactive fluids which, in absence of an electric field, exhibit Newtonian flow characteristics such that their shear rate is directly proportional to shear stress. However, when an electric field on the order of 10.sup.3 V/mm is applied, a yield stress phenomenon occurs such that no shearing takes place until the shear stress exceeds a yield value which rises with increasing electric field strength. The result can appear as an increase in apparent viscosity of several orders of magnitude. Thus, the electrorheological fluid "thickens" into a solid or semisolid condition where the particles of the fluid form into fibrillated, "pearl-chain" like structures between the electrodes producing the electric field. While electrorheological fluids are beneficial in providing for rapid and reversible response characteristics with typical response times being on the order of one millisecond, the shear stress limits of electrorheological fluids are constrained by the voltage potential and volume of fluid required for their adequate performance in known mechanical applications.
Another type of electroactive fluid is an electrophoretic or "electroseparatable" fluid. Electrophoretic fluids are suspensions similar to electrorheological fluids but are characterized by a very different response to an applied electric field. The particles within electrophoretic fluids exhibit a very strong electrophoretic migration. Rather than forming, in the presence of an electric field, a fibrillated structure that has an induced yield strength, electrophoretic fluids separate into particle-rich and particle-deficient phases by electrophoresis. The electrophoretic induced separation can produce much larger shear strengths at lower operating voltages than electrorheological fluids. Electrophoresis is a linear phenomenon with respect to electric field strength, while in contrast, the strength of an electrorheological fluid varies with the square of the electric field because of the dependence on induced dipole interactions for the electrorheological effect. Further, once electrophoretic induced separation is accomplished, the resulting shear strength of an electrophoretic fluid can be maintained under a reduced electric field.